[Ada] Improve error messages for dot notation when -gnatX not used
gcc/ada/
* einfo.ads (Direct_Primitive_Operations): Update the doc to
indicate that this field is used for all types now.
* sem_ch4.adb (Try_Object_Operation): Add parameter
Allow_Extensions set to True to pretend that extensions are
allowed.
* sem_ch4.ads: Same.
* sem_ch6.adb: Do not require Extensions_Allowed.
* sem_ch8.adb (Find_Selected_Component): Remove duplicate
"where" in comment. Improve the error messages regarding use of
prefixed calls.